1. Field of the Invention
The present invention relates to a needle-less injector, wherein a dose of liquid medicament is discharged in a thin jet at sufficient velocity to penetrate the epidermis of the human, plant or animal to be treated, thus to introduce the medicament into the tissues of the subject.
2. Description of the Prior Art
Needle-less injectors are uses as an alternative to hypodermic needle type injectors for delivery drugs, vaccines, local anaesthetics and other fluids into the tissues. The medicament is discharged in a jet at high velocity to first puncture the epidermis, and thereafter be deposited in the tissues of the subject. A variation is to press the discharge nozzle onto the epidermis and force the medicament at very high pressure through the epidermis.
Such injectors offer many potential benefits: because the hole made by the jet is smaller than that made by a needle, less pain is experienced by the patient when compared with hypodermic needle injection; the availability of the medicament to the capillaries is often enhanced because the spread of the medicament within the tissues is much greater than achieved with a needle injection, which deposits the medicament as a bolus at the needle tip; there is less chance of cross-contamination; animals, which are frequently uncooperative, are more easily injected, because there is no risk of a needle breaking or bending, and the injection is accomplished much faster than needle injection; there is no needle disposal problem; the risk of so-called needle-stick injury is eliminated.
Prior art devices typically use a spring-loaded piston pump to generate the injection pressure, in which the piston is retracted against a spring to withdraw fluid from a reservoir. At the end of the piston stroke (which may be adjustable) the piston is disengaged from the retracting mechanism and is urged suddenly by the spring to pressurise and discharge the fluid from the delivery nozzle. The retracting mechanism may be manual or motorised. In some devices, the piston is driven on the discharge stroke by gas or an electric motor instead of a spring.
Manually operated injectors generate a pressure in the medicament of about 100 bars. In operation, the discharge orifice is placed a small distance (about 10 mm) from the epidermis, and the high velocity jet strikes then penetrates the epidermis (free jet mode). The principle appears to be that the jet sacrifices some of its kinetic energy to puncture the epidermis, because if the nozzle is pressed firmly on the skin (contact mode), and the injector operated, the liquid is pressurised but has no kinetic energy, and is unable to pierce the skin. In the jet free mode, medicament is wasted, since some of the liquid is deflected sideways before puncture is completed, whilst in the contact mode, the epidermis deforms under the pressure of the liquid, which allows all of the medicament to escape without achieving penetration.
Powered injectors generate higher pressure--typically 600 bars or more, which is sufficient to penetrate the epidermis even when the discharge orifice is placed firmly on the skin (the contact mode). However, even in the contact mode a variable quantity of liquid is lost on each injection because the epidermis initially deforms before puncturing, and allows some liquid to escape. A further reason that the contact mode injectors leak is that often the orifice fitted to the injector is an artificial jewel of the type used for bearings in chronometers (because they are inexpensive, accurate, and efficient), but the method of mounting the jewel is such that the face of the orifice is always a small distance from the skin, and the consequent broadening of the jet results in lower force per unit area, and poor penetration.
The basic aim of all these devices is to apply the medicament with sufficient force to pierce the epidermis, but it is the rate of increase in force which is important rather than the nominal pressure used, and few prior art injectors can achieve a sufficiently high rate of pressure rise to ensure reliable and repeatable injections.
Laboratory tests on both manual and powered injectors often give encouraging results, but in practical situations, such as the vaccination of animals, very variable amounts are injected--frequently over 50% of the vaccine may be wasted, because of hairs and dirt on the skin, and movement of the animal. The difficulty in achieving successful injections is exacerbated if the subject does not co-operate, as in the case of animals, young children or elderly patients. Premature operation of the injector is common, as is relative movement between the injector orifice and epidermis which can cause tearing of the epidermis during injection. The amount of contact pressure applied varies between operators, and the action of releasing the trigger mechanism frequently results in a jerk of the injector just as it operates, again causing poor injections and poor reproducibility.
Various methods have been proposed to overcome these problems, although in the case of the free jet types, little can be achieved. Powered injectors frequently employ a vacuum device to suck the epidermis firmly onto the discharge orifice (see WO 82/02835--Cohen, and EP-A-347190--Finger) and thereby improve the seal between the orifice and the epidermis, and prevent relative movement. Alternatively, a pressure sensitive sleeve on the injector (see U.S. Pat. No. 3,859,996--Mizzy) is placed on the subject, whereby operation of the injector is prevented until the correct contact pressure between the orifice and skin is achieved.
Powered injectors have available a variety of sensing and control devices to enhance their performance, which are denied to manually powered injectors. However, they are frequently more complex and not easily adapted for portable use. The fact that they develop higher pressures than manual injectors means that their power consumption is high; gas powered injectors require a heavy cylinder of compressed gas, and electrically powered injectors are often mains powered; battery powered injectors require a heavy battery pack, and the limited availability or inconvenience of the power sources has meant that the use of powered injectors has been mainly confined to mass vaccination programs. In the case of batteries and gas cylinders, it is usually difficult to judge the number of operations available from power stored. Furthermore, the sensing methods used to enable optimum operation are invariably secondary or indirect. For example, U.S. Pat. No. 3,859,996--(Mizzy) discloses a controlled leak method to ensure the injector orifice is correctly placed at the required pressure on the subject's skin. When the placement conditions are met, the controlled leak is sealed off by contact with the subject's skin, and the pressure within the injector control circuitry rises until a pressure sensitive pilot valve opens high pressure gas to the drive piston. However, the actual pressure of the orifice on the skin is not being measured; a hair of dirt of other irregularity on the skin or sealing face of the orifice will prevent or retard the pressure rise in the control circuit, and the operator will unconsciously press the injector harder onto the skin. Also the timing characteristics may vary because of the said ineffective sealing, hysteresis of the pressure switch, and variations in the supply pressure. In other words, the parameters being measured are the effectiveness of the seal of the controlled leak sensor on the skin and the pilot valve response, not the actual pressure of the orifice on the epidermis. Still other devices us a sliding sleeve in contact with the subject's skin, whereby displacement of the sleeve is used to initiate the injection, but this method measures the load on the sleeve, not on the orifice as required.
It may be seen therefore that whilst needle-less injection potentially is more efficient than hypodermic needle injections for certain applications, the technique is very dependent on the ability of the operator and the compliance of the subject. Those injectors that have features designed to reduce these problems tend to be more complex and costly, and less portable. Furthermore, the simpler injectors designed for use by the patient are invariably complicated to load, clean, adjust and operate, and have not been particularly well designed to be "user friendly". For example, self injection of insulin by the diabetic patient has been an area of intense development, because such patients often have to inject themselves four time daily, and needle-less injectors offer the possibility of less pain and tissue damage. Nevertheless, the aforesaid variability in performance has prevented the widespread adoption of the technique, and the recommended cleaning and sterilization procedures are extremely inconvenient.
WO 93/03779 describes a needle-less injector of the present inventor, which aims to overcome, or at least mitigate, the problems described above. In summary, it provides a needle-less injector which comprises a chamber for containing liquid to be injected, the chamber being provided with a liquid outlet; a dispensing member movable in a first direction to reduce the volume of the chamber to cause liquid contained therein to be expelled through the said liquid outlet; and an impacting member arranged to strike the said dispensing member to cause movement thereof in the said first direction; the injector comprising a front portion which carries means defining said liquid outlet, a rear portion having means defining a handle for the injector, means for urging the rear portion away from the front portion, and means for actuating the injector, or permitting actuation thereof, in response to the movement of the rear portion towards the front portion against the force of the means urging them away from one another.
The injector described in WO 93/03779 has been found to give excellent results. The design described in detail therein is, however, essentially one intended for dispensing a large number of doses, say 1500, and uses a built-in electric motor and battery to provide power. There remains a need, which has not been adequately met by the prior art, for a needle-less injector which is suitable for dispensing a single dose, or a small number of doses, and which is sufficiently inexpensive that it can be disposed of thereafter.